After doing some research it is confirmed that radioactive thallium is not actually allowed in airports and would in fact set off radiation alarms. Your question about thallium in an aqueous solution contained in a glass bottle with a plastic top would also trigger the radiation alarms.

This would also show during the dexa scans if carried in person and the bottle itself would visually show on the xray belt as the bottle/bag it was in passed within the machine without a doubt.

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